The present invention relates to a chroma signal gain control circuit for use in a color television receiving circuit.
In a color television receiving circuit, if the level difference between the brightness signal and the carrier chroma signal takes place due to the fading or the mismatching of the antenna circuit, the chroma saturation factor (or simply the chroma) may sometimes be changed. Thus, an Automatic Chroma Control (ACC) circuit for automatically controlling the changes in the chroma saturation factor to hold the carrier chroma signal at a predetermined level has been typically used, as well known in the relevant art.
In this ACC circuit, the color burst signal level inserted at a predetermined level is extracted to control the gain of a band pass amplifier such that the color burst signal level may be constant. Therefore, an accurate chroma saturation factor can be attained by the aforementioned ACC circuit so long as the color burst signal and the chroma signal are supplied at a predetermined level ratio.
However, the level ratio of the color burst signal to the chroma signal is frequently changed due to noise in the broadcasting network system or the reinsertion of the color burst signal at a local station. Therefore, if the burst signal level is relatively attenuated, for example, the gain of the band pass amplifier is so controlled as to make that signal level constant, thus inviting a problem in that the gain for the chroma signal is so increased that the band pass amplifier is oversaturated to invite a distortion of the signal.
With this in mind, there has been disclosed in Japanese Laid-Open Patent Publication No. 49-57725 an ACC circuit having a first band pass amplifier, in which there is provided at the input of the first band pass amplifier an average value detecting circuit for detecting the average level of the input, and in which there is added a threshold circuit for judging when the output peak level of a second band pass amplifier becomes higher than the detected average level thereby to control the gain of the second band pass amplifier so that the output carrier chroma signal level may be constant, whereby the aforementioned disadvantage relating to over-saturation of the amplifier is obviated.
There is also proposed in Japanese Laid-Open Patent Publication No. 53-134325, another circuit in which the first and second band pass amplifiers are equipped with ACC circuits, respectively, with the ACC circuit of the second band pass amplifier serving to decrease the gain of a second amplified band pass output when this second amplified band pass output has either strong noises or an excessive signal level, whereby the aforementioned disadvantage relating to the over-saturation of the amplifier or the like is obviated.